Chapter 2 - Properties of Water/pH/Buffers (slideshare)
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Chapter 2 discusses the properties of water, including its polarity, hydrogen bonding, and role in solvation. It covers the concepts of acid-base equilibrium, including strong and weak acids/bases, the pH scale, and the importance of buffers in resisting pH changes. The Henderson-Hasselbalch equation is presented to describe the relationship between pH, pKa, and buffer concentrations.
![Ionization of Water
H20 + H20 H3O+
+ OH-
Keq= [H+
] [OH-
]
[H2O]
H20 H+
+ OH-
Keq=1.8 X 10-16
M
[H2O] = 55.5 M
[H2O] Keq = [H+
] [OH-
]
(1.8 X 10-16
M)(55.5 M ) = [H+
] [OH-
]
1.0 X 10-14
M2
= [H+
] [OH-
] = Kw
If [H+
]=[OH-
] then [H+
] = 1.0 X 10-7](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-14-320.jpg)
![pH Scale
Devised by Sorenson (1902)
[H+] can range from 1M and
1 X 10-14
M
using a log scale simplifies
notation
pH = -log [H+
]
Neutral pH = 7.0](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-15-320.jpg)
![Acid/conjugate base pairs
HA + H2O A-
+ H3O+
HA A-
+ H+
HA = acid ( donates H+
)(Bronstad Acid)
A-
= Conjugate base (accepts H+
)(Bronstad Base)
Ka = [H+
][A-
]
[HA]
Ka & pKa value describe tendency to
loose H+
large Ka = stronger acid
small Ka = weaker acid
pKa = - log Ka](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-17-320.jpg)
![Henderson-Hasselbach Equation
1) Ka = [H+
][A-
]
[HA]
2) [H+
] = Ka [HA]
[A-
]
3) -log[H+
] = -log Ka -log [HA]
[A-
]
4) -log[H+
] = -log Ka +log [A-
]
[HA]
5) pH = pKa +log [A-
]
[HA]
HA = weak acid
A-
= Conjugate base
* H-H equation describes
the relationship between
pH, pKa and buffer
concentration](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-21-320.jpg)
![Case where 10% acetate ion 90%
acetic acid
• pH = pKa + log10
[0.1 ]
¯¯¯¯¯¯¯¯¯¯
[0.9]
• pH = 4.76 + (-0.95)
• pH = 3.81](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-22-320.jpg)
![• pH = pKa + log10
[0.5 ]
¯¯¯¯¯¯¯¯¯¯
[0.5]
• pH = 4.76 + 0
• pH = 4.76 = pKa
Case where 50% acetate ion 50% acetic acid](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-23-320.jpg)
![• pH = pKa + log10
[0.9 ]
¯¯¯¯¯¯¯¯¯¯
[0.1]
• pH = 4.76 + 0.95
• pH = 5.71
Case where 90% acetate ion 10% acetic acid](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-24-320.jpg)
![• pH = pKa + log10
[0.99 ]
¯¯¯¯¯¯¯¯¯¯
[0.01]
• pH = 4.76 + 2.00
• pH = 6.76
• pH = pKa + log10
[0.01 ]
¯¯¯¯¯¯¯¯¯
[0.99]
• pH = 4.76 - 2.00
• pH = 2.76
Cases when buffering fails](https://image.slidesharecdn.com/chapter2-140419075421-phpapp02/85/Chapter-2-Properties-of-Water-pH-Buffers-slideshare-25-320.jpg)
Chapter 2 - Properties of Water/pH/Buffers (slideshare)
- 2. WHY DO WE NEED TO DO THIS AGAIN!!!
- 3. • Very polar • Oxygen is highly electronegative • H-bond donor and acceptor • High b.p., m.p., heat of vaporization, surface tension Properties of water
- 5. Water dissolves polar compounds solvation shell or hydration shell
- 6. Non-polar substances are insoluble in water Many lipids are amphipathic
- 8. Hydrogen Bonding of Water Crystal lattice of ice One H2O molecule can associate with 4 other H20 molecules •Ice: 4 H-bonds per water molecule •Water: 2.3 H-bonds per water molecule
- 12. Relative Bond Strengths Bond type kJ/mole H3C-CH3 88 H-H 104 Ionic 40 to 200 H-bond 2 - 20 Hydrophobic interaction 3 -10 van der Waals 0.4 - 4
- 14. Ionization of Water H20 + H20 H3O+ + OH- Keq= [H+ ] [OH- ] [H2O] H20 H+ + OH- Keq=1.8 X 10-16 M [H2O] = 55.5 M [H2O] Keq = [H+ ] [OH- ] (1.8 X 10-16 M)(55.5 M ) = [H+ ] [OH- ] 1.0 X 10-14 M2 = [H+ ] [OH- ] = Kw If [H+ ]=[OH- ] then [H+ ] = 1.0 X 10-7
- 15. pH Scale Devised by Sorenson (1902) [H+] can range from 1M and 1 X 10-14 M using a log scale simplifies notation pH = -log [H+ ] Neutral pH = 7.0
- 16. Weak Acids and Bases Equilibria •Strong acids / bases – disassociate completely •Weak acids / bases – disassociate only partially •Enzyme activity sensitive to pH • weak acid/bases play important role in protein structure/function
- 17. Acid/conjugate base pairs HA + H2O A- + H3O+ HA A- + H+ HA = acid ( donates H+ )(Bronstad Acid) A- = Conjugate base (accepts H+ )(Bronstad Base) Ka = [H+ ][A- ] [HA] Ka & pKa value describe tendency to loose H+ large Ka = stronger acid small Ka = weaker acid pKa = - log Ka
- 18. pKa values determined by titration
- 19. Phosphate has three ionizable H+ and three pKas
- 20. Buffers • Buffers are aqueous systems that resist changes in pH when small amounts of a strong acid or base are added. • A buffered system consist of a weak acid and its conjugate base. • The most effective buffering occurs at the region of minimum slope on a titration curve (i.e. around the pKa). • Buffers are effective at pHs that are within +/-1 pH unit of the pKa
- 21. Henderson-Hasselbach Equation 1) Ka = [H+ ][A- ] [HA] 2) [H+ ] = Ka [HA] [A- ] 3) -log[H+ ] = -log Ka -log [HA] [A- ] 4) -log[H+ ] = -log Ka +log [A- ] [HA] 5) pH = pKa +log [A- ] [HA] HA = weak acid A- = Conjugate base * H-H equation describes the relationship between pH, pKa and buffer concentration
- 22. Case where 10% acetate ion 90% acetic acid • pH = pKa + log10 [0.1 ] ¯¯¯¯¯¯¯¯¯¯ [0.9] • pH = 4.76 + (-0.95) • pH = 3.81
- 23. • pH = pKa + log10 [0.5 ] ¯¯¯¯¯¯¯¯¯¯ [0.5] • pH = 4.76 + 0 • pH = 4.76 = pKa Case where 50% acetate ion 50% acetic acid
- 24. • pH = pKa + log10 [0.9 ] ¯¯¯¯¯¯¯¯¯¯ [0.1] • pH = 4.76 + 0.95 • pH = 5.71 Case where 90% acetate ion 10% acetic acid
- 25. • pH = pKa + log10 [0.99 ] ¯¯¯¯¯¯¯¯¯¯ [0.01] • pH = 4.76 + 2.00 • pH = 6.76 • pH = pKa + log10 [0.01 ] ¯¯¯¯¯¯¯¯¯ [0.99] • pH = 4.76 - 2.00 • pH = 2.76 Cases when buffering fails